Heparin is a direct acting anticoagulant that has been used for the treatment of thromboembolic conditions for over 30 years. The major problems with therapy include overanticoagulation leading to bleeding episodes, and underanticoagulation resulting in thrombus formation. The incidence of these problems in hospitalized patients ranges from 5 to 20 percent. Rick factors for bleeding include age, dose, alcohol use, sex and renal function. Individualization of heparin therapy is currently based on empirical judgment. No method employing current computer technologies and pharmacokinetic and pharmacodynamic modeling techniques has been developed to assist in anticoagulating patients receiving heparin. Biotrack is developing products to assist in the management of patients with thromboembolic diseases. The first of these, Warfcalc, has already been developed and tested in patients treated with the oral anticoagulant, Warfarin. The purpose of this project is to extend Biotrack's expertise in developing expert systems for anticoagulation to heparin. This would involve developing a large population base for heparin which would define the best model for describing heparin's dynamics and kinetics. Biotrack proposes in Phase I to develop the population base for heparin and to develop a prototype expert system for predicting heparin response. In Phase II Biotrack would test prospectively the performance of such a program in a randomized clinical trial against standard medical therapy.